In some situations the surface flatness of objects, such as printed circuit boards, ceramic and metal substrates, papers, plastics, woven items and the like, is of special concern. For example, the manufacture of printed circuit boards and other electronic interconnection products is a multi-billion dollar global industry, and the flatness of these products is critical to their ability to undergo further manufacturing steps and to their ultimate reliability in operation as parts of computer, automobile, telecommunications, aerospace, military and other electronic systems. Non-flatness, or warpage, is a frequent problem in manufacturing due to inadequacies in design, materials, and/or processing of components, which are typically complex devices composed of several different materials. The ability to analyze surface flatness plays an important role in the designing, manufacture, processing and maintaining of objects for which surface flatness is an important characteristic.
Shadow moire measurement techniques are known in the field of photomechanics and have been applied previously to the measurement of warpage in printed circuit boards and other electronic packaging components. Examples of systems and methods for using shadow moire techniques for measuring the surface flatness of objects, such as printed circuit boards, are disclosed in U.S. patent application Ser. No. 08/778,214 and U.S. Pat. No. 5,601,364, both of which are incorporated herein by reference.
A shadow moire fringe pattern is created when the surface of an object is generally parallel to a grating, and the surface is viewed through the grating. The surface is illuminated through the grating so that a shadow of the grating is cast upon the surface of the object, and the shadow and the grating interact to create a shadow moire fringe pattern that is indicative of the warpage of the surface of the object. It is common to capture and analyze the image of the shadow moire fringe pattern. One prior conventional technique relies on counting the number of fringes within the image to quantify the flatness of the surface of the object. For many applications, this prior technique is very satisfactory. But the resolution possible with this prior technique is comparable to the distance between the grating lines. For example, for a 100 line-per-inch horizontal grating with an angle of illumination of 45.degree. relative to the vertical and angle of observation of 0.degree. relative to the vertical, the resolution of the analysis is 10 mils per fringe. This resolution is too large for many required measurements where a resolution below 1 mil is necessary for quality control or performance characterization.
After the acquisition of a shadow moire image that is indicative of the surface flatness of an object, it may be desirable to convert the image into a quantitative measurement of the out-of-plane displacement of the object at each point on the surface of the object. Using some conventional techniques of shadow moire analysis, this conversion step is slow and difficult. A single shadow moire image does not contain the information necessary to identify the maxima and minima in the surface contour, or the gradient of the contour at each point. Adding the required information to the analysis from other sources generally requires significant operator intervention.
It is known to use phase-shifting image processing to increase the sensitivity of shadow moire techniques and to automatically obtain an array that identifies the displacement of relative points on the surface of the object being examined. An example of a system and method that utilize shadow moire and phase-stepping image processing techniques to analyze the surface flatness of objects is disclosed in U.S. Pat. No. 5,307,152, which is incorporated herein by reference. Whereas the use of phase-shifting image processing with shadow moire techniques has provided advantages, it is believed that further improvements to systems and methods for measuring surface flatness using shadow moire technology and phase-stepping image processing would be further advantageous with respect to the design, manufacture, use and maintenance of articles of manufacture, such as printed circuit boards, ceramic and metal substrates, papers, plastics, woven items and the like.